GEOM namelist

One of the next releases of FDS will include the new GEOM namelist. Let's start experimenting on that, so that BlenderFDS will be ready when this feature is officially released.

More details on the subject are available on: geom_notes.tex

The new `GEOM` namelist

The namelist GEOM will describe one or more unstructured closed geometric surfaces that enclose solid portions of the fluid domain. These surfaces consist of a collection of triangular faces, where each face is built from three vertices. The user can assign a specific boundary condition to each of the faces.

A simple form of the GEOM namelist defining an unstructured solid is given by

&GEOM ID='UNSTRUCTURED_SOLID'
      SURF_ID='FIRE','INERT'
      VERTS= -1.0, -1.0,  0.0, 
              1.0, -1.0,  0.0, 
              0.0,  1.0,  0.0, 
              0.0,  0.0,  1.0,
      FACES= 1,3,2, 2,  
             1,4,3, 1,  
             3,4,2, 1,  
             2,4,1, 0,  
/

where ID specifies the solid name, in this case UNSTRUCTURED_SOLID, VERTS specifies the x_1,y_1,z_1, x_2,y_2,z_2, ... coordinates of the vertices, and FACES specifies a list of triangular faces and the SURF_ID that should be assigned to each of them.

Quality of the triangulated surface

In order for FDS to correctly detect the solid portion of the volume from the rest of the fluid domain, some tests are performed at start on the quality of the triangulated surfaces.

The folowing conditions enforced by FDS are checked before exporting by BlenderFDS on the collection of vertices and faces of each GEOM namelist:

Each vertex must be unique, shared by edges, be part of an only well-formed surface so that its neighbor ring of faces could in theory be continuously deformed to a disk: a vertex that abides to these conditions is called non-degenerate and manifold.
Each edge connecting vertices must have a non-zero length, and must always be shared by exactly two faces. An edge that abides to these conditions is called non-degenerate and manifold.
Each face cannot intersect the others and must have a non-zero area; such a face is called non-intersecting and non-degenerate.
The vertex orderings for the faces shall be chosen so that adjacent faces have outward consistent normals.
Each GEOM namelist may define one or more distinct volumes (unconnected volumes) that should never mutually intersect or self-intersect.
An upper limit is set to the number of triangles for each MESH cell; this condition reasonably limits the complexity of the discretization that can currently be elaborated by the immersed boundary method solver (This condition is not currently checked by BlenderFDS)

If the previous conditions are respected, a well-formed GEOM namelist is obtained that represents one or more closed, manifold, orientable, non intersecting triangulated surfaces, that enclose well-defined volumes.

BlenderFDS and GEOMs

When exporting to an FDS case, the Blender Object is transformed to one GEOM namelist. Its Blender Mesh geometry is triangulated, checked, and exported as it is to the corresponding VERTS and FACES parameters.

If you want to assign specific boundary conditions to the faces of the Blender Object, first go to Edit Mode. Then append a new Blender Material slot to the Object, select the faces, and click on Assign.

When you go back to Object Mode, click on Show FDS Code to see how the GEOM namelist is going to be exported:

! 2 surf_id, 86 vertices, 168 faces
&GEOM ID='Sphere' SURF_ID='INERT','Burner'
      VERTS=
            0.000000, 0.382683, 1.923880,
            0.000000, 0.707107, 1.707107,
            ...
      FACES=
            6,86,14, 1,
            4,13,5, 1,
            ...
/